Global Measurement of Nitrous Oxide Stable Isotopes Using Cavity Ring-Down Spectroscopy

Abstract:

Nitrous oxide continues to increase in the atmosphere mainly due to heightened microbial production from fertilized agricultural systems. Soil microorganism processes are spatiotemporally heterogeneous, limiting our ability to constrain the anthropogenic influence on N₂O production at a global scale. The intramolecular position of ¹⁵N (β position ¹⁵N¹⁴N¹⁶O versus α position ¹⁴N¹⁵N¹⁶O) in addition to δ¹⁵N^bulk-N₂O can aid in our understanding of both the biological controls and stratospheric influence of tropospheric N₂O. A subset of 22 sites from the NOAA Global Monitoring Division Cooperative Sampling Network is being measured in order to describe the global distribution and seasonality of N₂O isotopocules. Simultaneous and continuous measurement of N₂O mole fraction, δ¹⁵N^bulk-N₂O, δ¹⁵N^α-N₂O, and δ¹⁵N^β-N₂O is conducted using the Picarro G5101-i wavelength-scanned cavity ring-down spectrometer coupled with a quantum cascade laser capable of the mid-infrared wavelength detection needed for N₂O. While isotopic differences within and between sites are observed, long term measurement uncertainties of 0.7‰, 0.8‰, and 1.3‰ for δ¹⁵N^bulk, δ¹⁵N^α, and δ¹⁵N^β respectively, limit our ability to detect tropospheric trends. Applying additional correction factors for environmental conditions and molecular interference may help to reduce these uncertainties. Due to the lack of isotopic reference material for N₂O, we have developed an isotopic calibration technique using trace additions of δ¹⁵N^α and δ¹⁵N^β to our reference gas at the ambient mole fraction needed for laser based isotopic measurement.